# Squeeze rectangle texture to a triangle

I know how to stretch the texture by using UV coordinates, but I am trying to do the opposite:
I am trying to find a way to squeeze a rectangular texture to a triangle object:

[ATTACH=CONFIG]1375[/ATTACH]

Like in the image, move corner points 1 and 2 to the center and leave corner points 3 and 4 intact.

Is there some simple method of doing that?
Thank you!

To do what you want requires a custom texture-mapping formula in the fragment shader. Specifically, if the lower corners (labelled 3 and 4 in the diagram) have texture coordinates (0,1) and (1,1), and the apex corner has texture coordinates (0,0), then you can use e.g.

``````
uniform sampler2d tex;
in vec2 texcoord;
...
vec2 uv = vec2(texcoord.s / texcoord.t, texcoord.t);
vec4 color = texture(tex, uv);

``````

Note that the generated S coordinate is undefined at T=0, although that situation will only arise if the apex is exactly in the centre of a pixel. If that turns out to be an issue, add a very small positive constant to the divisor (or to the apex’ T coordinate).

With a projective mapping, both the S and T coordinates are divided by the interpolated Q coordinate (which is usually 1, but may be an affine function when performing texture projection, i.e. when using 4D texture coordinates). It isn’t possible to divide only the S coordinate, hence this can’t be implemented using fixed-function texture mapping (i.e. glEnable(GL_TEXTURE_2D)); you have to use a shader.

A similar approach can be used to map a rectangular portion of a texture to a trapezoid, although you may need to use the third coordinate § for the divisor if you don’t want it to converge at T=0.

It doesn’t work

[ATTACH=CONFIG]1376[/ATTACH]

and compare it with the changes from your code:
[ATTACH=CONFIG]1377[/ATTACH]

I devised the correct formula:

``````vec2 uv = vec2((texcoord.s - texcoord.t / 2.0) / (1 - texcoord.t), texcoord.t);
``````

This produces the following image:

[ATTACH=CONFIG]1378[/ATTACH]

Thank you GClements!

You wouldn’t know, as you didn’t try it.

Your texture coordinates are wrong. The apex corner has texture coordinates of (0.5,0) or (0.5,1) (probably the latter, as you’re dividing by 1-T). I was quite explicit about this:

[QUOTE=GClements;1282222]Specifically, if the lower corners (labelled 3 and 4 in the diagram) have texture coordinates (0,1) and (1,1), and the apex corner has texture coordinates (0,0), then you can use e.g.
[/QUOTE]